Free-machining stainless steel



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3,000,730 FREE-MACHINING STAINLESS STEEL Harry Tanczyn, Baltimore, Md, assignor to Armco Steel Corporation, a corporation of Ohio No Drawing. Filed Dec. '3, 1959, Ser. No. 856,926 4 Claims. (Cl. 75128) This invention is particularly concerned with stainless steels of the straight chromium free-machining grades, especially the well-known A.I.S.I. Type 416 of composition hereinafter set forth.

One object of my invention is to provide a hardenable stainless steel of the character indicated which may be readily tempered to a desired intermediate hardness value, this in close response to commercial specifications.

Another object is to provide a stainless steel of the general type described wherein the response to tempering treatment is made less sensitive and, at the same time, the machinability is improved, all at minimum cost and in simple, direct and reliable manner.

Other objects of my invention in part will be obvious and in part more fully pointed out during the course of the following disclosure.

The invention, accordingly, resides in the combination of elements, composition of ingredients, and in the relation of each of the same to one or more of the others, all as described herein, the scope of the application of which is indicated in the claims at the end of this speci fication.

To better understand my invention, it may be noted at this point that the straight chromium free-machining grades of heat hardenable stainless steels basically are of martensitic structure. They are comparatively inexpensive and of widespread utility. The A.I.S.I. Type 416 analyzes 0.15% max. carbon, 1.25% max. manganese, 1.00% max. silicon, 12.00% to 14.00% chromium, with .07% minimum of any one or more of phosphorus, sulphur and selenium, zirconium and molybdenum a maximum of 0.60%, and remainder iron.

The Type 416 steel is well suited for general use where good machining properties are required as well as re sistance to corrosion and/or to heat. It is the phosphorus, sulphur and selenium which imparts to the metal improved machining and non-seizing characteristics. Ready machinability admirably suits the steel for intricate machining operations. 1

Conventional heat-treatment includes a rapid cooling from a hardening temperature of about 1800 F. So treated, the steel displays a hardness in the approximate range of 43 to 44 Re. It is to be noted, however, that by suitable variations in a subsequent tempering treatment a rather wide range of mechanical properties may be developed. The tempering treatment is at somewhat lower temperature. This serves both to relieve stresses incident to hardening, and perhaps to .increase the toughness of the steel, While substantially retaining required hardness. It is this tempering which is relied upon to control the precise hardness of the steel for its intended purpose, and to bring the same to a selected value somewhat below its maximum of Ra 43-44 as noted above.

Actually, a customer frequently specifies, and this Within close limits, both the composition and the mechanical properties of the steel which he orders. For example, that the steel be processed at the mill to an intermediate hardness Within the narrow range, of approximately Rc 26-32. But in practice it is diflicult to bring the steel within such narrow limits. This is largely because of the rapid change of hardness which takes place when the steel is subjected to a tempering operation; the ameliorating effect is unduly dependent upon relatively small differences in the temperature at which the tempering operation is conducted. The steel is' much too sensitive.

rates @atenr In the present condition of the art, and in order to closely approximate the required hardness, it frequently is necessary to reheat the metal several times during the tempering process, and this by trial and error, first at one temperature and then at another, until the specified range of hardness is had. This practice is tedious and time-consuming. The difficulties are particularly severe when, as is usually the case, a large tonnage of steel in the form of sheet, strip, plate, bars, rods or wire is undergoing treatment as a batch. For here, uniform tem perature both within the batch and from one batch to another, is rendered diflicult, if not impossible. And so the response to tempering treatment varies substantially from batch to batch and from sheet to sheet, or plate to plate, etc. within the batch.

An important object of my invention, therefore, is to provide in direct and simple manner, with comparatively limited additional cost, a steel wherein the hardness of the free-machining grades of straight chromium stainless steel may be positively brought to selected intermediate values, and this in but a single tempering treatment, without unduly sacrificing toughness and resistance to impact.

Turning now to the practice of my invention, I find that the addition to the straight chromium free-machining grades of stainless steel, of comparatively small and closely-controlled quantities of columbium and/or vanadiurn will appreciably reduce the criticality of the change in hardness with variations of temperature in a stress-relieving tempering treatment. As a consequence, a broader range of tempering temperatures results, this assuring substantially the same hardness values in spite of differences in the tempering temperatures. These results are most surprising when it is recognized that although the prior art discloses many instances Where columbium has been added to stainless steel in substantial quantities, but in no instance have my advantageous results been obtained. In my steel the amount of columbium addition is highly critical, especially as it relates to the carbon content, all as appears more fully hereinafter.

Actually, of course, tantalum usually is employed along with columbium since the two occur together in the prealloy employed. The small amount of tantalum in sub stitution for part of the columbium additive, results in no appreciable diminution of the advantageous results had by columbium alone (that is, without an admixture of tantalum) as a constituent of the steel.

Columbium-tantalum introduced into the steel within the critical limits of my invention not only gives superior control over the intermediate hardness values realized through tempering but the addition also assures uniform hardness even with an appreciable spread in the temperatures actually attained in a batch of steel being treated.

Vanadium likewise improves the characteristics of temper hardness, as against the precise temperature of the tempering treatment, although vanadium exerts no beneficial effect on either the grain-size of the steel or the toughness. On the contrary, it seems to further promote brittleness and thus aid machinability particularly in matters of chip break-up. Best results are had with columbium and vanadium together (or columbium along with tantalum when combined with vanadium), because both the vanadium and columbium additives (or columbium together with tantalum) spreads the hardness control. The amount of vanadium used is about the same as the columbium (or the columbium along with tantalum).

Typically, in the production of the steel heretofore described, and in accordance with the practice of my invention, columbium-tantalum may be present in amount ranging from about 0.03% to about 0.35% of the steel,

3 with vanadium present in amount ranging from about 0.03% to about 0.50%.

Broadly, the steel of my invention essentially consists of about .07% to .14% carbon, .10% to 1.25% manganese, .07% to .50% sulphur and/or selenium, .10% to 1.00% silicon, 10.0% to 14.0% chronium, 1.00% max. nickel, 03% to 35% columbi-um and tantalum together, with or without vanadium in the amount of 03% to .50%, and remainder substantially all iron. Molybdenum, tungsten and nitrogen are present as residuals, the molybdenum and tungsten each in amounts up to about 20% and nitrogen in amounts up to about .05 For special purposes, however, molybdenum may be purposely added in amounts up to 0.60%, and zirconium in amounts up to 0.60%.

The free-machining stainless steel of my invention is hardened upon cooling from a temperature range of say about 1650" to 1950" F., enduring for about four hours. Cooling is had either in air or by quenching in oil or water. Following the hardening operation, the metal is then subjected to further heat treatment as suggested above, both for the relief of stresses and to lower the hardness of the metal. This is had by reheating the steel at a temperature between about 500 and 1200 F. and then cooling.

As specifically illustrative of the practice of my invention I give in Table I below the chemical composition of two steels according to my invention:

TABLE I Chemical analyses of two free-machining 12% chromium stainless steels The ingots of the two heats noted were rolled into coils of A round, then annealed and pickled. They were then leaded and drawn in three stages (.190", .170" and .150") and hardened at 1800 F. for /2 hr. and oil quenched. Following this they were tempered, the Heat 038199 at 1025 F. for 4 hrs. and air cooled and the Heat 038201 at 1020 F. for 2% hrs. and air cooled. Following this they were lined and finish drawn to size, the former .115/.l16 inch diameter and the latter .130/.132 inch diameter. Hardness readings were taken at the back, the center and the front of each coil. The wire of Heat 038199 amounted to values of Rockwell C27/ 29. That of Heat 038201 Rockwell (329/30. It is particularly noted that the hardness is substantially uniform throughout the coil.

In the free-machining steel of my invention there is achieved a consistency in the temper of hardness had in a batch of sheet, strip, wire, or the like, in spite of temperature differences which are reached throughout the batch during the tempering operation. While in a batch of commercial 416 stainless steel wire the tempered hardness was observed to range from about Rockwell 13C to Rockwell 20C, in my steel, in which vanadium and/ or columbium is present, the hardness variation from batch to batch was found to range only some one or two points on the Rockwell C scale.

The amount of columbium (or columbium plus tantalum) used in my free-machining steel is in every sense critical. For where the combined columbium-tantalum content and vanadium is lower than 03% no benefit is had. And where the additive is present in excess of 35% for colurnbium-tantal-um and in excess of .50% for the vanadium, or in excess of .50% for the combined addition, disappointing results usually areobserved; a loss in strength is noted. For best results the sum total of the columbium (or columbium-tantalum) and vanadium does not exceed about three times the carbon content of the steel. Even better results are obtained when this ratio is limited to not more than about one to two times the carbon content. With an excess of columbium-tantalum and/or vanadium there seems to be a tendency of these ingredients to tie up thecarbon within the metal, and to take the same out of solution. This sacrifices the hardenability of the metal, as well as its resultant hardness and strength following heattreatrnent.

It will be seen, therefore, that I provide in my inven tion a straight chromium free-machining grade of hardenable stainless steel in which there is had the-various objects hereinbefore set forth together with many prac: tical advantages. The steel of my invention reliably responds to a tempering treatment, which response changes less sharply with tempering temperature than the steel of the prior art.

All the foregoing, as well as many other practical advantages attend the practice of my invention.

Inasmuch as many modifications of the present embodiment of my invention will suggest themselves to those skilled in the art and that many embodiments of the underlying inventive thought will likewise come to mind, all falling within the scope of this disclosure, it is to be understood that I intend the foregoing description to be considered as simply illustrative, and not as comprising limitations.

I claim as my invention:

1. A quench-hardenable free-machining stainless steel in which the response to tempering treatment is broadened, said steel essentially consisting of carbon 07% to .14%, manganese .10% to 1.25 ingredient of the group sulphur and selenium in the amount of 07% to .50%, silicon .10% to 1.00%, chromium 10.0% to 14.0%, nickel maximum, ingredient of the group columbiumtantalum .03% to 35%, and vanadium 03% to .50%, molybdenum up to .60%, zirconium up to .60%, and the remainder substantially all iron.

2. A quench-hardenable free-machining stainless steel essentially consisting of carbon 07% to .14%, chromium 10.0% to 14.0%, ingredient of the group sulphur and selenium in the amount of .07% to .50%, columbiumtantalum 03% to 35%, vanadium 03% to .50%, with the columbium-tantalum together with vanadium amounting to about one to three times the carbon content, and the remainder substantially all iron.

3. A quench-hardenable free-machining stainless steel essentially consisting of carbon .07% to .14%, manganese .10% to 1.25%, silicon .10% to 1.00%,chromium 10.0% to 14.0%, nickel 1.00% maximum, ingredient of the group consisting of 07% to 50% sulphur and selenium, ingredient of the group consisting of colunibium-tantalum 03% to 35%, and vanadium .03% to .50%, the sum total of columbium-tantalum and vanadium additions hav ing a ratio to carbon of one to one up to about three to one, and the remainder substantially all iron.

4. A quench-hardenable free-machining stainless steel wherein temper hardness can be closely controlled within required value, said steel comprising about 10.0% to 14.0% chromium, about 0.07% to 0.14% carbon, about 0.10% to 1.25% manganese, about 001% to 050% phosphorus, about .07% to .50% sulphur, about 0.10% to 1.00% silicon, nickel up to about 1.00%, colurnbiumtantalum together with vanadium in sum total up to about three times the carbon content of the steel, the columbium-tantalurn being present in the amount of about 03% to 0.35% while the vanadium is present in quantity of about .03% to .50%, and the remainder substantially all iron.

References Cited in the file of this patent 

1. A QUENCH-HARDENABLE FREE-MACHINING STAINLESS STEEL IN WHICH THE RESPONSE TO TEMPERATING TREATMENT IS BROADENED, SAID STEEL ESSENTIALLY CONSISTING OF CARBON .07% TO .14% MANGANESE .10% TO 1.25%, INGREDIENT OF THE GROUP SULPHUR AND SELENIUM IN THE AMOUNT OF .07% TO .50%, SILICON .10% TO 1.00%, CHROMIUM 10.0% TO 14.0% NICKEL 1.00% MAXIMUM, INGREDIENT OF THE GROUP COLUMDIUMTANTALU, .03% TO .35%, AND VANADIUM .03% TO .50% MOLYBDENUM UO TO .60%, ZIRCONIUM UP TP .60%, AND THE REMAINDER SUBSTANTIALLY ALL IRON. 